Needle-free injectors are used as an alternative to needle-type hypodermic injectors for delivering liquid drugs and other substances through the skin and into the underlying tissues. The drug is dispensed from a drug capsule having a piston which is driven with sufficient force that the drug is expelled at sufficiently high pressure to pierce the skin. Typically, the drug capsule will comprise a hollow cylindrical chamber narrowing to a discharge orifice at one end, and a piston slidingly and sealingly located at the other. The piston is caused to move towards the orifice to dispense the drug by a ram, powered by a variety of means such as a spring, pressurised gas or pyrotechnic charge. The orifice diameter can vary from about 0.08 mm to about 0.7 mm, according to the application.
The more successful and controllable injectors employ a two-phase injection pressure profile; the first is a short but very high-pressure pulse to puncture the skin and the second is at a lower pressure to dispense the drug through the hole thus formed. Typically, the first pressure pulse will be of around 100 microsecond duration, and have a peak pressure of 300-500 bar, and the second will last for around 200 milliseconds with a pressure of around 100 bar. The duration of the second phase will vary according to the volume to be delivered.
It is highly preferred that the drug capsule is transparent, so that the contents may be checked for accuracy and contamination. This requirement has placed a severe limitation on the types of materials that may be used, because the transparent material must be strong enough to withstand the extremely high pressures, and must not adversely affect the drug. As a consequence, virtually all of the needle-free injectors proposed use a plastic drug capsule, typically made from polycarbonate. However, such materials are generally unsuitable for storing the drug, because they absorb water from the drug, or are permeable to oxygen, or react in some way with the drug. Therefore, drug capsules made from plastics are required to be filled immediately before use, a rather inconvenient procedure, with a risk of inaccurate filling and contamination, and requiring training of the operators.
The only material with a long history of satisfactory drug storage is borosilicate glass, but this is very brittle and hence there have been few injectors with glass capsules. The obvious problem with glass capsules is the potential for particles of glass to be ejected if they burst. The underlying causes of the weakness of glass capsules are tiny flaws which occur during manufacture, such as scratches and cracks.
The “Intraject” manufactured by Weston Medical Limited is a pre-filled single-use disposable needle-free injector, having one of the very few glass capsules suitable for long-term drug storage. This is a borosilicate drug capsule of up to 1 ml capacity, made to exceedingly close manufacturing specifications, and further improved by ion exchange strengthening. The breakage rate for these capsules is exceptionally low, but it is desirable to reduce this still further.
Several attempts have been made to reduce the breakage rate for these capsules. For example, further layers of material have been added to the capsule to provide increased physical strength (see international patent publication WO96/15821 in the name of Weston Medical Limited). However, this approach increases significantly the manufacturing costs of the capsule. Additionally, to still further reduce the breakage rate visual inspection techniques have been employed. However, visual inspection techniques are limited both due to the flaw-size which can be detected and the inherent difficulty in automation, which in turn leads to increased cost. Therefore, there is still a requirement for further reducing the incidence of breakages in a cost-effective manner.